External string buffers for generic-i/o-ports.

External string buffers for generic-i/o-ports.

Added allocate-buffer-bytes with default make-string value, to fluid
let to make-external-string when appropriate, i.e. NOT in the cold
load.  Not sure why it does not work in the cold load...

Hampered performance especially by replacing %substring-move! with
xsubstring-move!.  The former integrates unrolled loops.  Need a
%xsubstring-move! with all that, and open-coded
external-string-ref/set!.